Heavy crude oil is generally obtained from underground oil fields in combination with water. The water contains salts and other water soluble materials. At the production facility, the bulk of the water with dissolved salts is removed from the crude oil by gravity separation (i.e., settling) or by centrifuging. The emulsion might also be heated to 35-80.degree. C. to facilitate water separation. Residual water of 8% or less generally remains.
Removing the residual water is particularly difficult for crude oils which form water-in-oil emulsions with very small water droplet size that are stabilized with certain crude oil components. The problem is pronounced with heavy crude oils having API gravity of 30 or less, particularly those containing relatively high levels of natural surfactants, asphaltenes, and resin components. These components can stabilize water-in-oil emulsions, thereby making it more difficult to remove residual water from the emulsion.
API gravity is a measure of density defined by the American Petroleum Institute (API) as: EQU .degree.API=[141.5/specific gravity]-131.5
where the specific gravity is measured at 60.degree. F. API gravity is inversely proportional to density, i.e., the higher the API gravity the lower the density. Water has an API gravity of 10. Crude oil is generally less dense than water, with API gravity ranging from about 10 to about 60. Some heavy crudes have API gravity of less than 10, i.e., they are more dense than water.
Immiscible materials are more difficult to separate if their densities are similar. Consequently, separation of a water-in-oil emulsion is more difficult with a crude oil of API gravity in the lower half of the range, since the density of the oil approaches that of water.